1. Field of the Invention
This invention relates to a semiconductor device comprising a light detecting element and a periphery circuitry for discriminating an output level of the light detecting element, the element and the circuitry being formed integrally in a single semiconductor layer.
2. Prior Art of the Invention
There have been known a light detecting circuitry formed on a semiconductor substrate, as shown in Japanese Laid-Open Pat. No. 58-170077 for example.
In FIG. 1 showing such a conventional circuitry, a semiconductor light detecting integrated circuit comprises a photo diode 1 as a light receiving element, a load resistance 2 for converting the output current of the photo diode 1 into a voltage signal, a DC amplifier 3 for amplifying the light detecting signal of the photo diode 1 converted to the voltage by the load resistance 2, a bias circuit 4 for setting the bias level for the DC amplifier 3 and a Schmitt trigger circuit 5 for converting the output signal of the DC amplifier 3 into a two-valued or binary signal using a reference signal thereof.
The photo diode 1 may be formed using a bipolar structure in which a pn junction is formed between a p-type silicon substrate and an n-type epitaxial layer. The resistance 2 has several hundreds .OMEGA. to one mege .OMEGA. and is formed by implantation of boron ion into the n-epitaxial layer as a sheet resistance having about 5 K.OMEGA./cm.sup.2.
However, in such a conventional circuit, the output current of the photo diode is converted into a voltage signal by the high resistance and further amplified by the DC amplifier, thereby providing a two-valued output. Therefore, the conventional circuitry requires a large number of cells such as 40-50 elements and occupies a large area on the semiconductor substrate. Especially the problem is the large occupied area of the high resistance on the semiconductor chip. For instance, a 500 K .OMEGA. resistance requires a large area of about 150 .mu.m.times.300 .mu.m.
Furthermore, as the output current of the photo diode is converted to a DC voltage signal using the high resistance and amplified by the DC amplifier to produce a two-valued or binary output signal, the bias level is simultaneously amplified during the amplification, making the sensitivity of the circuitry fluctuating and the characteristics of the circuitry greatly depending on the temperature thereof.